This invention relates to a fuse for use in an electronic circuit and in particular to an EPROM cell containing a floating gate for providing a fuse between two portions of a circuit.
A fuse is a device which in response to a signal applied thereto can be converted from one state to another state irreversibly. Typically, a fuse comprises a material which normally conducts current but which can be converted to an open circuit by passing a current pulse through the material so as to heat and destroy the material. It is also known in the art to use floating gate transistors as fuses. Floating gate transistor fuses typically comprise a source and drain region separated by a channel region. A floating gate is formed over the channel region and a control gate is formed over the floating gate. By varying the charge on the floating gate, the threshold voltage required to allow a current to flow between the source and the drain (i.e., to turn on the device) is controlled.
It is known in the art to include floating gate transistor fuses in EPROM circuits which include redundant cells. In such a structure, the fuses permit malfunctioning EPROM cells to be decoupled from an array of EPROM cells and redundant cells to be coupled to the array to replace the malfunctioning cells. A typical floating gate transistor fuse is covered by a metal cover which blocks light from striking the floating gate within the fuse so that the state of the fuse does not change while the rest of the EPROM is being erased. Such a structure is described by Alan C. Folmsbee in a paper entitled "PROM Cell Made With An EPROM Process", published in the International Electron Device Meeting Technical Digest in 1983, incorporated herein by reference.
Most of the periphery of the Folmsbee metal cover directly contacts the silicon surrounding the floating gate transistor in order to minimize the amount of light reaching the floating gate while the remainder of the metal cover is separated from the underlying silicon by an insulating layer of 1.85 micron thick silicon dioxide.
Unfortunately, the Folmsbee structure requires a large surface area, in part because of a large opening in Folmsbee's light blocking metal cover. This opening is necessary to permit access to the drain and control gate of the floating gate transistor. Since the opening is large, it permits a large amount of light to enter underneath the metal cover. In order to reach the Folmsbee floating gate, this light would have to pass through the silicon dioxide insulation layer and repetitively reflect off the metal cover and underlying silicon. (See Folmsbee FIG. 1). Each time the light reflects off the silicon, a portion of the light is absorbed. This absorption prevents the light underneath the metal cover from reaching the floating gate. To permit a large percentage of the light under the metal cover to be absorbed before reaching the floating gate, Folmsbee places his floating gate about 80 microns from the opening.